Omnidirectional antennas are known. One illustrative example form of omnidirectional antenna is so called indoor antennas which have a multiband capability and preferably transmit and receive with a vertical polarization. They have a base plate on which an antenna element which is in the form of a monopole projects transversely from, that is to say at right angles to, the base plate. The entire arrangement is generally covered by means of a protective housing (radome).
In such example antennas, a recess is generally incorporated in the center, or slightly offset in the vicinity of the center, on the base plate. Generally, the base plate is metallic or at least conductive, and generally has a circular shape in a plan view. In the recess, a plug element for a plug connection is generally anchored Generally, the plug element includes a contact element in the form of a plug (i.e., male connector). A coaxial cable in the form of a second plug element, generally in the form of a plug element in the form of a female connector, can generally be connected there, from underneath. The outer conductor in this case makes contact with the base plate. The inner conductor of the feed cable is electrically connected via the plug contact that is provided on the base plate to the antenna element. The antenna element is generally in the form of a monopole and projects from the base plate. The inner conductor is electrically conductively isolated from the base plate, and thus from the outer conductor of a coaxial cable to be connected.
Omnidirectional antennas such as these may be designed such that they can transmit and receive simultaneously in two or more frequency ranges and/or simultaneously in two or more frequency bands.
Indoor omnidirectional antennas of this type have previously been produced and marketed by Kathrein-Werke K G. By way of example, these indoor omnidirectional antennas can transmit and receive simultaneously in the following frequency ranges:
824960MHz14251710MHz17101880MHz18501990MHz19202170MHz
Antennas with a multiband capability are likewise known, are produced and marketed by Kathrein-Werke and, for example, can be operated simultaneously at the following frequencies:
876890MHz890960MHz17102170MHz21702500MHz
While much work has been done in the past in connection with the design of indoor omnidirectional multiband antennas, further improvements are possible and desirable.
The technology herein provides a physically comparatively very small antenna which has a multiband capability, that is to say it has a very broad bandwidth overall, and which can also be used as an omnidirectional antenna. The antenna technology herein is able to operate simultaneously over even wider bandwidths.
It must be regarded as very surprising that a greatly widened bandwidth for simultaneous operation in widely differing frequency ranges is possible by means of a single physically very small antenna. This is accomplished, in exemplary illustrative non-limiting example implementations herein, by the antenna being fed by means of a series or capacitive line coupling at the “foot point” of the antenna element.
Very wide bandwidths are possible in this way. By way of example, the antenna can be operated without any problems simultaneously in the 800 to 1000 MHz band, in the 1400–3500 MHz band, or else, for example, in the 5000 to 6000 MHz band. Owing to the series (capacitive) line coupling, there may be resonances in the upper band.
One exemplary illustrative non-limiting implementation of an antenna, which is like a monopole, is preferably rotationally symmetrical, or is preferably designed to be rotationally symmetrical at least in specific angle ranges. It can have at least one section which widens conically in the longitudinal direction of the antenna. This section can be in the form of a monopole. The antenna may also be designed such that only its external shape is conical, overall.
The antenna may thus in principle also be radially symmetrical or may transmit and receive symmetrically, that is to say it may have a cross sectional shape such that the antenna can be made to be coincident when it is rotated through a specific angle in a plane about a central axis. This may, for example, apply solely to the antenna element or, for example, solely to the base plate, or to both.
Alternatively, the antenna element, which is in the form of a monopole or is similar to a monopole, may be cylindrical.
The antenna element, which is in the form of a monopole, of the antenna is preferably in a form, however, which is subdivided into a first section, which widens conically away from the base plate, and a cylindrical second section, which is adjacent to it. In other words, the antenna element is preferably formed from a combination of a conical antenna element section and a cylindrical antenna element section. The conical part of the antenna element primarily acts as a monopole for the upper frequency bands. The cylindrical part of the antenna element, in contrast, interacts with the associated counterweight surface (base plate) more for the lower frequencies. As a positive feature, it should be noted that this means that no reaction can be found from the cylindrical part on the upper frequency bands.
The series and/or capacitive line coupling may comprise a series and/or capacitive inner conductor coupling. Such coupling is preferably provided via a first coupling part, connected to the feed line (inner conductor of a coaxial conductor), in the form of a rod that projects from the base plate and is isolated from the base plate. The second coupling part, which is coupled to it, is connected to the antenna element, or is part of the antenna element. The second coupling part is preferably tubular. In particular, in order to achieve protection against rotation, the coupling part may also be in the form of a polygon or the like, that is to say, for example, it may have an n sided polygonal cross section.
In general terms, the cross sectional shape may be designed such that it has at least one shape that is not circular. This allows the antenna element, which is similar to a monopole and is formed from a combination of a conical surface and a cylindrical section adjacent to it, to be fitted by means of its internal tubular section (which projects from the foot point of the antenna element) directly onto the first coupling part, which is in the form of a rod and is connected to the feed cable. Since the first and second coupling parts, that is to say the feed line and the antenna element which is in the form of a monopole, are conductively isolated in order to produce series line coupling, an isolating sleeve of the first coupling part is preferably fitted, onto which the second coupling part of the antenna element, which is in the form of a monopole, can be fitted.
This also results in very simple assembly and installation, since the antenna element can be mounted, without any soldering and just by pushing it on, above the base plate on the first coupling part, which is connected to the feed line, and with the interposition of an insulating isolator.
However, the isolator need not necessarily be composed, for example, of a plastic material with a dielectric constant which can be selected in advance. Air may also be used as an isolator. In this case, all that is necessary is to use a suitable centering device and/or spacer in order to ensure that the fitted antenna element cannot make an electrically conductive contact with the coupling part under discussion, which is in the form of a rod and projects from the base plate, and/or with the base plate itself.
The series feed also makes it possible to minimize the antenna element's height in comparison with the conventional solution. This also makes it possible to reduce the counterweight area (base plate), thus making it possible to achieve a comparatively small physical size.